The present invention relates to a method for recovering precious metals from metallurgical products such as ores, wastes and electrolytic slimes and, more particularly, to an improved fire refining precious metal assay method for these materials.
The need for separating precious metals from materials such as ores is well-known. As discussed in U.S. Pat. No. 4,643,365, there are long known methods in the prior art for processing different types of precious-metal containing materials using lead as a collector for the precious metals recovered. Thus, in U.S. Pat. No. 718,087 silver and gold are recovered from their ores in a two-stage process comprising reverberatory smelting with molten lead. U.S. Pat. No. 815,851 teaches smelting hydrometallurgical products containing precious metals with litharge and reductant and cupelling the formed lead precious metals alloy product. U.S. Pat. No. 415,526 discloses a process for separating Se and other elements of the sulphur group and noble metals from each other by fusion with a collector, such as lead, for the noble metals. GB-A-689824 discloses extraction of precious metals from ashes or residues, for example, goldsmith's ashes and copper electrolysis sludge using a lead compound. GB-A-1574274 relates to smelting of waste products in a blast furnace with lead oxide and coke to form a lead smelt containing any precious metals. The above patents are hereby incorporated by reference.
In the fire refining assay process the precious metal containing material is mixed with a flux containing litharge (lead oxide), a reducing source, usually a carbon material, and fused in a furnace. At the temperature of the furnace, the carbon monoxide given off by the carbon source reduces the lead oxides to form metallic lead which rains down through the molten mass and acts as a solvent and collector for the precious metals. The mixture is poured into a mold to cool, forming a lead-precious metal alloy button and a slag. The slag is removed leaving the button which is then cupelled at about 954.degree. C. to absorb the lead and/or the drive off the lead as lead oxide leaving the precious metals which can then be conveniently assayed by, e.g., gravimetric means.
Collectors other than lead have been used in the prior art. U.S. Pat. No. 1,896,807 discloses the use of copper and iron and U.S. Pat. No. 2,048,152 discloses the use of nonmetals silicon, phosphorous and boron. Copper, nickel and iron are used as collectors in U.S. Pat. Nos. 4,448,604 and 4,451,290.
In "Notes on Assaying and Metallurgical Laboratory Experiments" by R. W. Lodge, John Wiley and Sons, 1906, pages 47 and 48, a different assaying process called scorification is described wherein bismuth is suggested as the only metal that could be used to take the place of lead in the assaying of ores for silver. It is noted though that silver losses are very high and other problems are encountered when using bismuth.
While the prior art methods of fire refining assaying are useful, the need still exists for more efficient methods of assaying and, in particular, for environmentally safe methods which do not use lead as the collector metal.
It is an object of the present invention to provide an environmentally safe method for the recovering of precious metals from precious metal containing materials.
It is a further object of this invention to provide an environmentally safe method for the fire refining precious metal assay method.
Other objects and advantages will become apparent for the following detailed description.